1. Field of the Invention
This invention relates generally to mechanical face seals and more particularly, to a mechanical face seal arrangement which is used to seal fluids, especially fluids which may be harmful to the environment, with minimum fluid leakage.
2. Background Art
Mechanical face seals have been used for sealing a variety of devices in which a fluid is sealed in a housing and the housing has an aperture through which a relatively rotating shaft extends. Sealing of the devices is necessary to contain the fluid being sealed which, in certain cases, may present harmful or even dangerous effects to the environment surrounding the sealed housing. In those cases, emissions resulting from fluid leakage must be eliminated as much as possible. A mechanism for maintaining zero emissions regardless of the performance of any one particular component of a seal system is desirable, and preferably, a mechanism for eliminating any hazardous process fluid flow even if one or more of the mechanical face seals of a system fail altogether.
Several dual seal arrangements are described in U.S. Pat. No. 4,290,611 issued to J. Sedy and assigned to a common assignee as the present invention. The dual seal arrangements described therein provide various mechanisms by which the seals maintain leakage from the housing to a minimum. However, none of the dual seal arrangements illustrated in U.S. Pat. No. 4,290,611 is intended for sealing hazardous or environmentally detrimental fluids, and consequently, the seal systems described therein are not adapted to anticipate a catastrophic seal failure of both mechanical face seals. Thus, if both of the seals, or in certain cases, the outboard seals, fail in those systems, at least a small amount of the process fluid is expected to leak into the environment.
A dual seal system in which a buffer fluid is utilized to provide a barrier to leakage of a process fluid is described in U.S. Pat. No. 5,249,812, also assigned to a common assignee as the present invention. U.S. Pat. No. 5,249,812 describes a seal system providing a buffer fluid, such as oil or other liquid lubricant, into a buffer chamber between the two seals of a dual seal system. One feature of this invention is that the mechanical seal face has spiral grooves which pump buffer fluid "upstream" against the process fluid pressure, so that some buffer fluid is injected by the pumping grooves into the process fluid.
The mixing of the sealed process fluid and the buffer fluid is not desirable for specific combinations, and may not be desirable in any event for a specific class of sealed process fluids.
Thus, what is necessary is a dual seal system, or a plural seal system, which provides the features of maintaining zero emission of the sealed process fluid into the ambient environment while simultaneously avoiding the injection of the buffer fluid into the process fluid and evacuating any mixture of process fluid and buffer fluid to a safe area removed from the seal where the mixture can be further processed.